Question 8: What is thermocouple? Explain the working of thermocouple by drawing its diagram. By sketching curve explain the variation in thermoelectric emf with temperature.

Thermocouple is pair of wires of two dissimilar metals that convert heat energy directly into electrical energy.

In 1821, Thomas Seebeck performed an experiment by joining the two wires to make a circuit. He placed one junction of the wires at constant temperature of 0oC and the other in a beaker whose temperature was gradually raised by burning a flame beneath it. A galvanometer was provided in the circuit to observe the current. At the same temperature of the junctions, the galvanometer showed no deflection. However when the temperature of one junction was gradually increased, he noticed the current increasing with the temperature. This is called Seebeck Effect. He kept on increasing the temperature of one end. The current also increased to a maximum value. After that maximum value, the current began to decrease with further increase in the temperature.

The temperature of the hot junction when the current is maximum is called Neutral temperature. Neutral temperature is independent of the temperature of the cold junction.

Thermocouples are low cost and simple in use. Therefore, they are widely used in the industries.

Variation of thermoelectric emf with temperature

In the figure above, a simple arrangement is shown for the study of thermocouple. It is a Cu-Fe thermocouple. Junction B is at 0oC (ice) and junction A is at high temperature. When the temperatures of both junctions are same, there is no thermo emf. However, when the temperature is slowly increased at junction A, the galvanometer shows a deflection.

Thermo emf increases with temperature until it is maximum for a certain value of the temperature. If temperature is further increased, thermo emf decreases. The temperature at which thermo emf is maximum is called neutral temperature, Tn. The value of neutral temperature is constant for a particular thermocouple and depends upon the nature of the material used. When the thermo emf starts decreasing with temperature after Tn, it becomes zero at a certain value of temperature. That temperature is called inversion temperature.

The adjacent graph shows variation of thermo emf with temperature of the hot junction. This particular thermocouple has its Tn at 518oF and inversion temperature at 1004oF.